There is substantial interest in Group III-nitride materials and devices due to applications in optoelectronics, photovoltaics, and lighting. Group III-nitrides are composed of nitrogen (N) in combination with one or more elements from Group III of the Periodic Table of the Elements: i.e., boron (B), aluminum (Al), gallium (Ga), and/or indium (In). Some examples of Group III-nitrides include GaN, GaxAl1-xN, GaxIn1-xN, GaxAlyIn1-x-yN, MN, and InxGa1-xN, where 0≦x≦1.0 and 0≦y≦1.0. Light emitting diodes (LEDs) incorporating the semiconducting nitrides of Al, Ga, and In can be tuned to emit light over the entire visible range with appropriate choices of layer stack structure, composition, and doping, a fact that makes such materials very important to the solid state lighting industry.
For many applications, it is desirable to transfer Group III-nitride-based structures (e.g., Group III-nitride-based films and/or devices) from the substrate on which the Group III-nitride materials were grown to a handle or carrier substrate. Such transfer steps may be needed to allow additional layers (such as contacts and/or reflectors) to be formed on a bottom (substrate) side of the Group III-nitride film stack; to free an expensive growth substrate for reuse; or to provide a substrate that is more transparent and/or flexible than the growth substrate. Recently, there has been renewed interest in being able to transfer GaN-based films and/or GaN-based LED devices grown on Si to alternative carrier substrates.
Laser lift-off (LLO) is a well known technique for releasing GaN-based (and more generally, Group III-nitride-based) layers from sapphire growth substrates. In this process, pulsed UV laser radiation is directed through the transparent (non-absorbing) sapphire substrate to the sapphire/GaN interface. The GaN that is closest to the sapphire/GaN interface strongly absorbs the laser radiation, heats up, and decomposes into gaseous N2 and a residual film of metallic Ga, producing a degradation in adhesion. As described by C. R. Miskys et al., “Freestanding GaN-substrates and devices,” Physica Status Solidi C0 1627 (2003), this LLO technique may be applied over large (e.g., 50 mm diameter) areas to produce thick (approximately 300 μm thick), freestanding GaN layers. The above-described LLO process has also been used to produce thin (a few μm to 60 μm thick) GaN layers, in a configuration in which the GaN layer is bonded to a layer of material serving as a mechanical support prior to the laser irradiation.
The options are more limited for releasing GaN-based layers grown on Si, since LLO as described above is not suitable for detaching GaN-based films and/or devices from non-transparent growth substrates such as Si. Two main approaches have been used: In a first approach, a handle or carrier substrate is bonded to the top of a GaN layer stack, followed by Si substrate removal by grinding and etchback [see, for example, M. Lesecq et al., “High Performance of AlGaN/GaN HEMTs Reported on Adhesive Tape,” IEEE Elect. Dev. Lett. 32 143 (2011)]. In a second approach, see, for example, Rogers, et al., “Unusual strategies for using indium gallium nitride grown on silicon (111) for solid-state lighting,” Proc. Nat. Acad. Sci. 108 10072 (2011), the boundaries of tile-shaped InGaN regions to be transferred are defined by deep trenches that are etched through the InGaN and into the Si substrate. Using the trenches for access, the Si under the GaN tiles is then removed with a wet etch process that undercuts the Si with a fast lateral etch rate, allowing the tiles to be released and transferred as desired.
However, it would be useful to have simpler and less costly methods for transferring GaN-based films and devices from a Si growth substrate to alternative substrates or carriers.
There are also situations in which it would be useful to have an inexpensive and reliable method for patterning GaN-based structures disposed on Si substrates in order to create patterned GaN-based structures surrounded by relatively wide (e.g., 0.1 to 3 mm) border regions of bare Si. For these large feature sizes, resolution is not an issue; the main requirements for such a patterning process are good edge definition and minimal Si removal.